For pre-upsetting of a head on an elongate blank it is known to place a wire blank in a die having a bottom stop, e.g. in the form of an ejector pin, following which the blank is formed by means of a punch in a pre-upsetter.
To obtain optimum quality of the head and to avoid deflection of the blank during the upsetting process, it is of decisive importance that the free length of the wire blank between the die and the pre-upsetter is sufficiently small. Since, however, a large volume in the head is frequently desired, this distance is usually increased to the maximum length. It is frequently desired at the same time that pre-upsetting proceeds to a great diameter, which increases the load on the pre-upsetter pin. These circumstances limit the maximum upsetting ratio that can be achieved, said upsetting ratio being the length of the wire outside the retention of the die divided by the wire diameter. It is desirable to achieve an upsetting ratio as great as possible.
It is known to improve the upsetting ratio by allowing the pre-upsetter to move away from the die during the process. This results in the short distance at the start of the process, while the length is increased with a simultaneous corresponding increase of the diameter during the process, whereby the blank remains stable. This takes place in the prior art by spring loading the pre-upsetter so that it is pressed away from the die as the head is formed. This prior art is mentioned e.g. by Billigmann/Feldmann: "Stauchen und Pressen", 1973. However, this process has the drawback that it is difficult to optimize the process, because the movement of the pre-upsetter away from the die is only controlled by a spring, and this restricts the size of upsetting ratios that can be achieved.
In this prior art, the punch follows the forced movement of the forming mechanism from e.g. a crank mechanism.